Hydroformylation is a well-known process in which an olefin is reacted with carbon monoxide and hydrogen in the presence of a catalyst to form aldehydes and/or alcohols containing one carbon atom more than the feed olefin. It is also known as the Oxo process, or as the oxonation process. Cobalt is a preferred catalyst for the high pressure hydroformylation of C5-C14 olefinic feedstocks, in particular those that are rich in branched and internal olefins. The cobalt carbonyl catalyst typically produces oxygenated product mixtures that are richer in the usually more desired less branched isomers, as compared to the carbonyl catalysts of other suitable metals, in particular of rhodium.
The present invention is concerned with the recovery of cobalt carbonyl catalyst from the hydroformylation reaction, also known as the Oxo or oxonation reaction.
The starting liquids that are involved in high pressure hydroformylation comprise olefins which may be mixtures of olefins such as those obtained from olefin oligomerisation units. For example the olefins may be mixtures of C5 to C12 olefins obtained by the phosphoric acid or zeolite catalysed oligomerisation of mainly C3 and C4 olefins and mixtures thereof. C5 olefins may also be present during oligomerisation, as well as traces of ethylene. Where olefin mixtures are used as feed for hydroformylation, they may have been fractionated to obtain relatively narrow boiling cut mixtures of mostly the appropriate carbon number for the production of aldehydes and alcohols with the desired carbon number. Alternatively the olefins may be obtained by other oligomerisation techniques. Such techniques include the dimerisation or trimerisation of butene using a nickel based or nickel oxide catalyst, like the Octol® process or the process described in U.S. Pat. No. 6,437,170. Others include oligomerisation processes for ethylene, propylene, pentenes and/or butenes, preferably single carbon number feedstocks and more preferably the unbranched, even more preferably terminal olefins such as butene-1, using nickel salt and involving di-alkyl aluminium halides, like the range of Dimersol® processes. Yet other oligomerisation processes may employ a zeolite or a molecular sieve oligomerisation catalyst for the oligomerisation of propylene and/or butenes and/or pentenes. The olefin products of these processes are typically branched and contain relatively low amounts of linear olefin isomers, typically less than 10% wt. The olefins may also be obtained from ethylene growth processes, such as the Shell Higher Olefins Process (SHOP) or the Ziegler process, in which case they are often straight chain, preferably terminal olefins, and are called linear alpha olefins or normal alpha olefins. The SHOP process may include a metathesis step, in which case also uneven carbon numbers may be produced. The olefins from ethylene growth may have C6, C8, C10 or C12, or even higher carbon numbers such as up to C14, C16, C18 or even C20, or they can be mixtures obtained from the Fischer-Tropsch process for the conversion of synthesis gas to hydrocarbons and which generates olefins of a range of carbon numbers, primarily containing terminal olefins but which may show some side branches along their longest alkyl chain, and which may also contain some internal olefins, linear and branched. In this case, also the higher carbon numbers may be useful starting liquids. Fischer-Tropsch olefins suitable for high pressure hydroformylation are disclosed in EP 835 234, but many other disclosures in this field may readily be found. The Fischer-Tropsch process uses syngas as the starting material, and suitable sources thereof are disclosed hereinafter.
The starting materials for the olefin oligomerisation processes mentioned above may be obtained from fluid catalytic cracking (FCC), from the steam or thermal cracking of gasses such as ethane and propane, of liquids such as liquefied petroleum gasses (LPG), of naphtha, of gasoil or heavier distillate, or even of whole crude. The oligomerisation starting material may also come from oxygenate-to-olefin processes, and from paraffin dehydrogenation processes.
The gases that are involved in the hydroformylation reaction include carbon monoxide and hydrogen, frequently supplied in a mixture that is known as synthesis gas or “syngas”. Syngas can be obtained through the use of partial oxidation technology (PDX), or steam reforming (SR), or a combination thereof that is often referred to as autothermal reforming (ATR). Thanks to the water-gas-shift reaction for supplying the hydrogen, it can be generated from almost every carbon containing source material, including methane, natural gas, ethane, petroleum condensates like propane and/or butane, naphtha or other light boiling hydrocarbon liquids, gasoline or distillate-like petroleum liquids, but also including heavier oils and byproducts from various processes including hydroformylation, and even from coal and other solid materials like biomass and waste plastics, as long as these provide a carbon source and can be brought into the reaction zone. When using liquid feeds for syngas generation, a steam reformer may involve a pre-reformer to convert part of the feed to methane or other light hydrocarbon gasses before entering the actual reformer reaction. The use of coal as feedstock for generating syngas is well known, preferably via the POX or ATR route. Such syngas may be fed directly as syngas feed for hydroformylation, but also as a feed to a Fischer-Tropsch process to generate the olefin feedstocks for the hydroformylation reaction. The latter is of interest for geographic regions where the other above mentioned carbon sources, in particular oil and gas, are less abundant.
The syngas is typically present in the hydroformylation reaction in a stoichiometric excess. Upon completion of the hydroformylation (oxonation) reaction, typically a separate gas phase is present, and in addition a significant amount of gasses becomes dissolved in the liquid reaction mixture. In combination, these comprise the unreacted gaseous reactants and any gaseous inerts that may have entered with the reactants and/or the catalyst. These excess gasses are typically separated off in a high pressure separator and/or after flashing the reaction product to a lower pressure.
The high pressure offgasses may contain entrained liquid and cobalt carbonyls, because of non-ideal separations at such high pressures, and it is proposed in U.S. Pat. No. 2,667,514 and GB 660,737 to include a scrubber on the high pressure offgas to scrub the offgas of these entrained species.
When vaporous aldehydes having three to five carbon atoms are produced, the offgasses may contain significant amounts of aldehydes. It may therefore be advantageous to employ the technique disclosed in U.S. Pat. No. 3,455,091, or in W. J. Scheidmeir, “Hydroformylierung von Butenen und Pentenen—Synthesen, Produkte und Möglichkeiten ihres Einsatzes”, Chemiker-Zeitung, 96e Jahrgang (1972) Nr. 7, pp 383-387, in which the offgasses may be scrubbed with water or with a high boiling solvent, such that the C3-C5 aldehydes may be recovered.
After completion of the oxonation reaction, the metal catalyst must be removed from the reaction products because it is typically undesired in any downstream processing, such as hydrogenation.
The cobalt species that is believed to be the active form of the catalyst for hydroformylation is a carbonyl compound, typically hydr(id)ocobalt (tetra)carbonyl, HCo(CO)4. Under the reaction conditions of high temperature and hydrogen partial pressure, it is believed that the following equilibrium reaction occurs, and that the equilibrium is significantly shifted to the left.2HCo(CO)4<--------->Co2(CO)8+H2  (1)
The hydroformylation catalyst is typically homogeneous, hence remains in the organic product of the olefin hydroformylation reaction. Co2(CO)8 is typically soluble only in an organic medium, such as the organic hydroformylation product. HCo(CO)4, however, is more versatile. It is also able to move to a water phase, if present, and dissociate as a Brönsted acid, and it has a vapour pressure, such that, at lower pressures, it may also move into a separate gas or vapour phase, if present.
Several technologies for recovery and recycle of a cobalt catalyst from the hydroformylation reaction are known.
Many of the cobalt recovery technologies comprise the conversion of the cobalt carbonyls to a water soluble salt, a step that is typically performed at a pressure that is significantly below the hydroformylation reaction pressure. Dissolved gasses separate at lower pressures from the reaction product, and volatile HCo(CO)4 may therefore be present when these gasses are separated as offgasses from the hydroformylation reaction product.
When cobalt carbonyls are converted to a water soluble Co2+ salt, carbon monoxide is liberated, typically resulting in a separate gas phase that may be separated as an offgas stream. Several of these cobalt carbonyl conversion techniques employ in addition an oxygen-containing gas as an oxidant. Air is typically used, and the air may be diluted, to address flammability concerns, with nitrogen and/or another gaseous diluent. These additional gasses further increase the amount of offgasses from the process. As the cobalt conversion in these techniques may not be complete, their offgasses may also contain minor amounts of volatile cobalt. These cobalt conversion techniques are suitably called “airless demet” when no extra oxidant is introduced, and “air demet” if an extra oxidant is used. This nomenclature is also used when air is not employed as the source of the oxidant. These cobalt conversion techniques may be preceded by a cobalt carbonyl extraction step, such as in the process disclosed in our copending patent application with attorney docket number 2008EM222. In particular the air demet technique is found to be most suitable to combine with the extraction step, because of the higher volume and energetic efficiency.
Some of the known cobalt recovery technologies make use of the volatility of HCo(CO)4. It is proposed in J. Falbe, “Carbon Monoxide in Organic Synthesis” (1970), to recover volatile cobalt carbonyls from offgases, by washing the offgas either with fresh olefin or with solvents or oil. The cobalt containing olefin or solvent is then used for charging the catalyst to the hydroformylation reaction. With lower olefins, such as ethylene, propylene and butenes, GB 702,192 and GB 702,222 propose the reaction medium for offgas scrubbing under pressure, so that unreacted olefin may also be recovered and returned to the reaction.
The so-called “Cobalt Flash” process employs a low pressure stripping step to remove a major part of the cobalt catalyst from an organic cobalt-containing reaction product into a stripping gas. The volatile cobalt is subsequently absorbed from the cobalt containing stripping gas in a suitable solvent, such as the feed olefin, and recycled to the oxonation reaction. This “Cobalt Flash” process is disclosed in more detail in U.S. Pat. No. 4,625,067 (Hanin) and its many variations in U.S. Pat. Nos. 5,235,112, 5,237,104, 5,237,105, 5,336,473, 5,410,090, 5,457,240, and 7,081,553, and in European Patent 643 683 or WO 93/24436. The stripping gas, from which the volatile cobalt has been removed, is then preferably recycled by a blower or low-head compressor to the low pressure stripping step to again pick up volatile cobalt. The solvent into which the volatile cobalt was recovered by absorption, is in all the Cobalt Flash processes routed to the hydroformylation reaction, because it constitutes the major source of cobalt catalyst for the reaction. An improved cobalt absorption step is disclosed in U.S. Pat. No. 5,354,908, offering a more concentrated cobalt containing olefin stream for feeding to the hydroformylation reaction. The “Cobalt Flash” process may comprise a secondary cobalt conversion or recovery step, such as those described before, to increase the effectiveness and/or efficiency of the overall process, and wherein the remainder of the cobalt that is not stripped may then primarily be recovered, and preferably recycled, typically in a non-volatile form. The air demet technique is very suitable for this purpose, and a combination of the “Cobalt Flash” technique with the two step decobalting process as described in our co-pending patent application U.S. Ser. No. 61/092,833 was found to be highly suitable for obtaining a high level of cobalt removal and recovery from the organic hydroformylation reaction product.
In any of these cobalt removal and recovery processes as described, in particular in any so-called air demet or airless demet step that is included, or in steps wherein the pressure is let down from the high pressure hydroformylation pressure to a lower pressure, low pressure offgas streams may be generated that contain volatile cobalt carbonyls, more particularly HCo(CO)4. When oxygenates are produced that have 6 or more carbon numbers, these streams typically contain only small amounts of oxygenates, so that extra steps and equipment for their recovery is typically not included.
We have found however that the volatile cobalt contained in these offgasses may create problems downstream. These offgasses may be compressed, for recycle to the hydroformylation reaction or for enabling a more commercially advantageous disposition, or they may be disposed of as such, alone or in combination with gasses from other sources, and typically as fuel gas for combustion in a furnace through a burner device. We have found that the HCo(CO)4 may form solid cobalt deposits in the equipment downstream from the offgas separation, in particular in a compressor, or a control valve, or a burner device but also in the piping itself. The deposits may be in the form of dicobaltoctacarbonyl and/or as cobalt clusters and/or as cobalt metal. These deposits typically foul the equipment at undesired locations and tend to impair the proper functioning thereof, in particular of any compressor, burner, or heat exchanger that is exposed to these offgasses, and/or increase the need for maintenance interventions.
In many circumstances, a volatile cobalt absorption step, such as provided in the “Cobalt Flash” technique and which serves to capture the cobalt into a liquid that is then pumped up to high pressure and routed to the hydroformylation reaction, may not be available as part of the entire process, or the pressure of the cobalt containing offgas may be insufficient for routing it to the absorption step. Providing an additional absorption step including associated equipment to recycle the cobalt-containing absorption liquid as an extra catalyst charge to the hydroformylation reaction may bring an extra process complexity and additional investment that may not be desirable in view of the small amount of cobalt catalyst that may be recovered from the offgasses. Because of the high pressures cobalt hydroformylation processes typically operate, the cobalt-containing absorption liquid would have to be pumped up to those high pressures in order to allow its introduction into the hydroformylation reactor, which represents a significant extra investment burden.
There remains therefore a need for removing volatile cobalt from an offgas that is separated from an organic cobalt-containing hydroformylation product in a simple, effective way, by a method that is volume and energy-efficient. The current invention provides a solution to this need.